The drive unit of motor vehicles (engine and power transmission train) is increasingly subjected to regulating and control mechanisms in order to optimize the interplay of an engine and its associated transmission. For instance, to reduce fuel consumption in motor vehicles, it will be necessary not only to improve engine control systems but also to adapt transmission ratios. To achieve this, automatic transmissions present themselves as a solution, with continuously variable transmissions (CVTs) in particular being increasingly used. A CVT allows continuous transmission adjustment until the required transmission ratio has been set. By means of a transmission element (chain, band, belt, etc) located between the discs of two disc assemblies, the required transmission ratio can be continuously adapted. By applying the same contact pressure values on these two disc assemblies, which corresponds to the so-called pretensioning force, the force level for this transmission element is adjusted. Specifying different values for this contact pressure on the two disc assemblies creates an additional transport force component for moving this transmission element. By varying this contact pressure, and thus the transport force, the transmission ratio may be continuously varied in any number of steps.
The regulating processes employed in continuously variable transmissions normally use transmission or speed controllers to effect control. The transmission input or drive speed is controlled by means of a control circuit designed as a speed controller. This transmission input or drive speed represents the product from transmission output speed, which is proportional to vehicle velocity, or drive speed (disturbance variable), and transmission ratio (control variable). To this end, the control circuit features a summation element at the controller input. As an augend, this summation element is given the nominal value of the transmission input speed (reference magnitude), and the addend is provided by the actual value of the transmission input speed (controlled variable). The control deviation, which is obtained at the output of the summation element by calculating the difference between the two quantities nominal value of the transmission input speed and actual value of the transmission input speed, is fed to a control element consisting of a proportional-action-controller, limiter, and lowpass filter. The output of this control element then supplies the time derivative (gradient) of the transmission input speed. In order to improve control response, the speed controller can be provided with (post-connected to) a second control circuit of acceleration control type, where the time derivative (gradient) of the transmission input speed provided as a nominal value by the control output of the speed controller represents the reference magnitude. In other words, in addition to speed control, a speed gradient control is also effected.
The disadvantage of these regulating processes for controlling continuously variable transmissions is that the control element of the speed controller, which control element generates the nominal value of the transmission input speed gradient, based on the control deviation (the speed difference of the transmission input speeds), and thus determines the control response, features a constant proportional constituent, that is, the controlling speed, or controlling rate, of the transmission input speed, and thus the control response is always the same value (irrespective of the relevant nominal value specified for the transmission input speed). Therefore, this always specifies the same control response so that it is not possible to influence (even though this might often be desirable) the output quantity at the control output of the speed controller, or the reference magnitude of the speed gradient control in relation to the nominal value specified for the transmission input speed, which in turn will affect driving safety or ride comfort.